The present application relates to ballasts, or power supply circuits for gas discharge lamps. It finds particular application for use with current fed instant and/or rapid start electronic ballasts or power supply circuits and will be described with particular reference thereto. It is to be appreciated, however, that the present application is also applicable to other controllers, and is not limited to the aforementioned use.
Presently there are two prevailing starting methods for starting gas discharge lamps. One is instant start, and the other is rapid start. In both the instant start and rapid start methods, cathodes of the lamp are pre-heated before lamp ignition. With an instant start electronic ballast, the cathodes are preheated by a glow discharge current. This is the current that goes through the lamp before the lamp ignites. Typically, the voltage potential of the glow discharge current is high, and can range between approximately 400 and 500 Vrms or more. In the preheating phase, when the lamp is not conducting, such high potential differences can cause bombardment of the cathodes, resulting in some of the physical material of the cathode sputtering off each time the lamp is lit. Thus, lamps that utilize the instant start method of ignition tend not to have as long lives as lamps that utilize the rapid start method. Typically, a lamp that uses instant start will last about 80% as long as the same type of lamp using rapid start.
With a rapid start electronic ballast, the cathodes are pre-heated with a separate voltage on the cathodes, while maintaining low voltage across the lamp. Therefore, the glow discharge current is low, being less than about 10 ma in comparison with instant start circuits. In the rapid start mode, the time that high voltage potentials across the lamp are applied without the lamp conducting is significantly reduced during start-up, and the bombardment of the cathodes does not occur to the same extent as with the instant start method, significantly extending lamp life.
There is a drawback, however, to using the rapid start mode. Presently, once a preheating current is applied, it is generally not removed from the cathodes, even after the lamp ignites. Resultantly, while the lamp is lit, the low voltage heating current is continuously applied on the cathodes. Thus, lamps that utilize the rapid start method to start are consuming more power than lamps that use the instant start method. With a single lamp, it is likely that up to about 1.5 extra Watts of power will be consumed, and with a three lamp ballast, it is likely that between 4.5 and 6 extra Watts of power will be consumed. This extra power is consumed (simply producing heat) without producing any added light output, that is, without producing extra lumens. Thus, the trade-off from instant start to rapid start is greater lamp life for added power consumption.